1. Field of the Invention
The present invention relates to a radio communication terminal to be operated by a battery, and more specifically to a radio communication terminal capable of increasing a usable time of the terminal.
2. Description of the Related Art
In recent years, cellular phone terminals (each of which hereinbelow will be alternatively referred to as a “terminal”) contain a variety of applications, so that while various ways of usage are made possible, the usable time thereof is required to be increased as long as possible. Such terminals operate with a secondary battery working as a power source, so that the following three points are important to increase the usable time of the terminal: enhancement of the battery capacity, lowering of the end voltage, and enhancement of the efficiency of a terminal circuit.
(End Voltage)
A terminal of the type described above contains a power amplifier to generate transmission power for a base station. During a communication event, power consumption of the power amplifier is dominant, so that efficiency enhancement of the power amplifier is important to enhance an actual-usage communication time (i.e., battery life). Generally, the battery voltage is used as it is for a power source of the power amplifier in the case of the maximum transmission power.
On the other hand, most circuits other than the power amplifier of the terminal are configured of CMOS integrated circuits. The power supply voltage required for the circuits is sufficiently low in comparison to the battery voltage, so that the power supply voltage is supplied after having been converted to a low voltage by, for example, a DC/DC converter or series regulator. For some portions, particularly, a display section or the like requiring high voltage, a step-up DC/DC converter, for example, is used.
The operable voltage of the terminal is determined by a voltage range wherein the power amplifier is operable, so that a lower limit voltage (“end voltage,” hereafter) at which the power amplifier is operable is important.
A circuit of a power amplifier used in a previous terminal is optimized to exhibit high efficiency at about 3.5 V that is an average discharge voltage of lithium ion secondary battery. In this case, the end voltage is set to a range of from about 3.3 V to about 3.2 V, which is a lower-limit power source voltage enabling satisfying wireless or radio characteristics standards of the terminal by using the power amplifier.
A continual wait time of a terminal is determined by a time necessary for the battery voltage to reach the end voltage in the receiving operation. A continual communication time of the terminal is determined by a time necessary for the battery voltage to reach the end voltage in a predetermined communication state (transmit or receive operation). However, a threshold value to be practically used is different depending on the operation state, as described below.
(Operation at End Voltage)
According to the previous terminal, in the event a single type of the end voltage is stored, when the battery voltage reaches the end voltage, if the state is a communication state, a communication termination process according to a protocol is performed. For example, the termination process for saving data and the like of the terminal, and then the power source is turned OFF. The single type of the end voltage indicates that one type of a threshold is used for a battery voltage that is to be detected in the state where the value power amplifier is operating.
However, a battery voltage being detected is such that, due to influences of, for example, internal resistances of the battery and wiring resistances inside of the terminal, the current flowing to internal resistors and the voltage drop level are different depending on the operation state (during wait operation, communication, or activation of a respective application, for example). This causes a problem for performing the detection of the battery voltage at all times by using a constant threshold value. As such, a certain number of threshold values are provided and compared corresponding to the operation state, thereby to select a single end voltage.
(Capacity Enhancement of Battery and Voltage Reduction of End Voltage)
FIG. 1 shows example discharge load characteristics of a lithium ion secondary battery. As can be seen from the drawing, the battery voltage slowly decreases as discharge proceeds, and sharply decreases at a certain point.
In recent years, since a cellular terminal contains a variety of applications, the terminal is required to have enhanced in, for example, capacity or capacity density, and development for new electrodes is now in progress. However, as shown in FIG. 2, according to the discharge load characteristics of a large-capacity battery, the drop of the battery voltage tends to increase as discharging increases. As such, to efficiently use features of the large capacity battery, it is necessary to reduce the end voltage of the terminal and to improve the circuit power efficiency in a wide range with respect to fluctuations in the battery voltage.
(Efficiency Enhancement of Circuits of Terminal)
FIG. 3 shows a configuration example of a transmit section of the previous terminal.
An antenna 1 is a component that performs transmission or reception of radio waves. A duplexer 4 is a component that separates transmission waves and reception waves that are different in frequency from one another, thereby to enable transmission and reception to be synchronously performed through the antenna 1. A band pass filter 6 is a component that removes unnecessary out-of-band noise from a transmission modulation wave, thereby to pass the transmission wave to a power amplifier 2. The voltage of a battery 3 is supplied to a power source of the power amplifier 2 through a switch 5. The switch 5 is controlled to ON only for effecting transmission, but is controlled to OFF in, for example, a power OFF time and reception-wait time of the terminal, thereby to prevent the battery from being unnecessarily consumed due to leak current. For the switch 5, a p-channel MOSFET having an ON-resistance of several tens of megohms (mΩ), for example, is used, wherein when the power amplifier 2 outputs a maximum transmission power, a voltage drop of a several tens of millivolts (mV) occurs.
It is prerequisite that the power amplifier 2 operates all time with the voltage of a battery 3, FET (field effect transistor) sizes and matching circuits are optimized to improve the efficiency at an average discharge voltage of the battery under the maximum transmission power. Depending on the case, an isolator (not shown) is interposed between the duplexer 4 and the power amplifier 2. In a CDMA (code division multiple access) terminal, precise power control is performed for the power amplifiers.
FIG. 5 shows investigation results of output power distributions in urban areas, wherein the rate of using a transmission power exceeding 22 dBm is 0.2% or lower, and an average output is 3.5 mW (5.4 dBm).
For cellular phone terminals, although IS-95 of USA TIA standard specifications, ARIB STDT-53, and the like specify the transmission maximum power to be 200 mW, it is most likely to be 10 mW or less during a practical use time.
FIG. 4 is a configuration example of a transmit section used in a CDMA terminal. In the configuration, when, as described above, a power smaller than a power maximum transmission power is handled, a DC/DC converter 7 is used to enhance the efficiency by reducing the power supply voltage of a power amplifier 2. More specifically, the output voltage of the DC/DC converter 7 is varied corresponding to the transmission power, whereby high efficiency power amplification is implemented in a wide range of the transmission power.
In this case also, a largest possible power source voltage needs to be secured under the maximum transmission power. As such, a method is employed to cause the DC/DC converter 7 to produce an output voltage substantially the same as that of the battery. Alternatively, a switch 5 shown in FIG. 3 and the DC/DC converter 7 shown in FIG. 4 are parallel coupled together, wherein the switch 5 is turned ON only under the maximum transmission power.
By way of another previous example, Japanese Unexamined Patent Application Publication No. 2001-217661 discloses power amplifiers used in a CDMA terminal employing a scheme that uses a switch to switch the number of power amplifiers depending on the output power. In this case, as shown in FIG. 3, the battery voltage is directly used for the power source of the power amplifiers.
FIG. 6 shows a virtual load line (A-class virtual load line is shown to simplify the description) of the power amplifier. In this case, the tilt of the load line is Imax/(Vknee−Vmax), which corresponds to the reciprocal of a load resistance of a transistor.
A power Po that the transistor generates isPo=(⅛)(Vmax−Vknee)Imax; anda DC input power Pdc isPdc={(Vmax+Vknee)Imax}/4.A drain efficiency η isη=Po/Pdc=(½)*(Vmax−Vknee)/(Vmax+Vknee).
The above indicates that as the power source voltage is lower, the magnitude of the knee voltage (Vknee) becomes non-negligible to the extent of deteriorating the efficiency.
The above teaches that although the power source voltage of the power amplifier under the maximum transmission power can be reduced by increasing the tilt of the load line shown in FIG. 6, the efficiency practically attainable is deteriorated.